Resistance welding of an end cap for nuclear fuel rods

ABSTRACT

An open end of a cladding tube of a nuclear fuel rod is plugged with an end plug having a main body and a cylindrical bond portion extending from the main body and terminating in a cladding seat with reduced diameter compared with the cylindrical bond portion. The plugging includes clamping the open end of the cladding tube against the cladding seat of the end plug and, while clamping, applying electrical current between the end plug and the open end of the cladding tube so as to force the open end of the cladding tube over the cladding seat and slide over the cylindrical bond portion of the end plug and to generate a resistance weld between a cylindrical bonding surface of the cylindrical bond portion of the end plug and the inside surface of the open end of the cladding tube.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/712,742, filed on Dec. 12, 2012, now U.S. Pat. No. 9,922,731, whichclaims the benefit of U.S. Provisional Application No. 61/625,410, filedon Apr. 17, 2012, the entire disclosures of which are incorporated byreference herein.

BACKGROUND

The following relates to the welding arts, nuclear reactor arts, nuclearfuel rod arts, nuclear power generation arts, and related arts.

In a typical nuclear reactor, the reactor core generally includes anumber of fuel assemblies each of which is made up of an array of fuelrods. Each fuel rod includes a tubular cladding containing fuel pelletscomprising fissile material. The cladding is sealed by upper and lowerend caps or plugs. The nuclear reactor core is made up of an array ofsuch fuel rods, and is disposed in a pressure vessel containing primarycoolant (typically water, although heavy water or another coolant isalso contemplated). The primary coolant flows through the nuclearreactor core and is heated by the radioactive core. In a typical boilingwater reactor (BWR) configuration, the heated coolant boils to formprimary coolant steam that is piped out of the pressure vessel and usedto drive a turbine. In a typical pressurized water reactor (PWR)configuration, the primary coolant remains in a subcooled state and ispiped through steam generators located outside of the vessel to heatsecondary coolant that drives a turbine. In a variant integral PWRconfiguration, the steam generators are located inside the pressurevessel and the secondary coolant is pumped into the steam generators.

In general, each fuel rod includes a column of nuclear fuel pelletsloaded into a cladding tube, and end plugs secured to opposite (e.g.,bottom and top) ends of the tube. The end plugs should provide areliable seal to prevent leakage of primary coolant into the fuel rods.In known approaches, the top and bottom end plugs are girth or buttwelded to the opposite ends of the tube, for example by fusion weldingor solid state welding.

Resistance welding of the end plugs to the cladding is also knownwherein a cladding tube is butted against an end plug. In this approach,a high current is passed between the cladding and the end plug which iscompressively loaded. Resistance at the interface between the end plugand the cladding generates localized heating resulting in a diffusionbond. While resistance welding has many desirable attributes, theprocess has some shortcomings. For example, non-destructive weldexamination is generally not feasible. Bond quality can also besusceptible to some contaminates, in some cases, with no means ofdetection. Weld upset, or flash, typically must be mechanically removedor suppressed in a post-weld process that complicates the processing.

SUMMARY

In accordance with one aspect, a method comprises plugging an open endof a cladding tube of a nuclear fuel rod with an end plug having a mainbody and a cylindrical bond portion extending from the main body andterminating in a cladding seat with reduced diameter compared with thecylindrical bond portion. The plugging includes clamping the open end ofthe cladding tube against the cladding seat of the end plug and, whileclamping, applying electrical current between the end plug and the openend of the cladding tube. The combination of the clamping and theapplied electric current is effective to force the open end of thecladding tube over the cladding seat and to slide over the cylindricalbond portion of the end plug and to generate a resistance weld between acylindrical bonding surface of the cylindrical bond portion of the endplug and the inside surface of the open end of the cladding tube. Insome embodiments the cladding seat includes a cylindrical seat portionof reduced diameter compared with the cylindrical bond portion and anabrupt or gradual annular step between the cylindrical seat portion ofthe end plug and the cylindrical bond portion of the end plug. In someembodiments the annular step is about 0.002 inches. In other embodimentsthe annular step is greater than 0.002 inches. In yet other embodimentsthe annular step is not utilized. In some embodiments the cladding seatcomprises a chamfered or frustoconical end of the cylindrical bondportion of the end plug. In some embodiments the end plug includes aslide stop defined between the main body of the end plug and thecylindrical bond portion of the end plug, the slide of the open end ofthe cladding tube over the cylindrical bond portion of the end plugbeing stopped by the slide stop. For example, the slide stop may be anabrupt or gradual annular step between the cylindrical bond portion ofthe end plug and the main body of the end plug. In some suchembodiments, the slide stop includes an annular groove that receivesbuildup material displaced during the slide of the open end of thecladding tube over the cylindrical bond portion of the end plug. Inother embodiments the distance of travel is controlled by a travellength process.

In accordance with another aspect, a method includes operations as setforth in the immediately preceding paragraph, and further includes theoperation of loading fuel pellets comprising fissile material into thecladding tube. The method may further include repeating the loading andthe plugging for each of a plurality of cladding tubes to generate aplurality of assembled nuclear fuel rods, and constructing a fuelassembly comprising an array of the assembled nuclear fuel rods.

In accordance with another aspect, a nuclear fuel rod includes: an endplug having a cylindrical bond portion, a tapered tip at one end of theend plug, and a seat portion of reduced diameter compared with thecylindrical bond portion at the opposite end of the end plug; a claddingtube having an end plugged by the cylindrical bond portion of the endplug with the seat portion disposed inside the plugged end of thecladding tube and the tapered tip extending outside of the plugged endof the cladding tube; and a weld bonding the cylindrical bond portion ofthe end plug and the inside surface of the plugged end of the claddingtube. In some embodiments the seat portion comprises a cylindrical seatportion of reduced diameter compared with the cylindrical bond portion.In some embodiments seat portion comprises a chamfered or frustoconicalend of the cylindrical bond portion of the end plug. In some embodimentsthe end plug further has a cylindrical main body disposed between thecylindrical bond portion and the tapered tip, the cylindrical main bodyextending outside of the plugged end of the cladding tube andterminating in the tapered tip. In some such embodiments the cylindricalmain body has a larger diameter than the cylindrical bond portion and anabrupt or gradual annular step is defined between the cylindrical mainbody of the end plug and the cylindrical bond portion of the end plug.In some such embodiments the end plug further has an annular groove inthe cylindrical bond portion at the abrupt or gradual annular stepdefined between the cylindrical main body and the cylindrical bondportion. The end plug optionally further includes a stub protruding fromthe seat portion, the stub not contacting the cladding tube.

In accordance with another aspect, a nuclear fuel rod as set forth inthe immediately preceding paragraph further includes a stack of fuelpellets comprising fissile material disposed in the cladding tube. Anapparatus may include a pressure vessel containing a nuclear reactorcore comprising an array of such nuclear fuel rods immersed in primarycoolant water.

In accordance with another aspect, an end plug for plugging an end of anuclear fuel rod cladding tube, the end plug includes: a tapered tip atone end of the end plug; a cylindrical bond portion sized to form aninterference fit inside the end of the nuclear fuel rod cladding tube;and a seat portion of reduced diameter compared with the cylindricalbond portion at the opposite end of the end plug from the tapered tip.In some embodiments the end plug further includes a cylindrical mainbody of diameter D₁ disposed between the tapered tip and the cylindricalbond portion, and the cylindrical bond portion has diameter D₂ which isless than diameter D₁ of the cylindrical main body. In some suchembodiments the seat portion comprises a cylindrical seat portion havingdiameter D₃ which is less than diameter D₂ of the cylindrical bondportion, while in other such embodiments the seat portion comprises achamfered or frustoconical end of the cylindrical seat portion.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various process operations and arrangements ofprocess operations. The drawings are only for purposes of illustratingpreferred embodiments and are not to be construed as limiting theinvention.

FIG. 1 diagrammatically shows an illustrative fuel rod including upperand lower end plugs.

FIG. 2 diagrammatically shows a nuclear reactor core comprising fuelrods as shown in FIG. 1.

FIG. 3 diagrammatically shows a nuclear reactor including the nuclearreactor core of FIG. 2.

FIG. 4 shows a sectional view of an end plug with a fuel rod claddingseated for resistance welding.

FIG. 5 shows a sectional view of the end plug/cladding of FIG. 4 afterperforming the resistance welding.

FIGS. 6-9 show cross-sectional views of additional end plug embodimentssuitable for resistance welding to a fuel rod cladding as disclosedherein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, a fuel rod 2 is diagrammatically shown. Thefuel rod 2 includes a cladding tube 4 containing a stack of fuel pellets6 comprising fissile material. The cladding tube is plugged at its upperend with an upper end plug 7, and is plugged at its lower end with a(lower) end plug 8. The plugging of the lower end by a lower end plug 8,or the upper end by an upper end plug 7 includes a resistance weld asdisclosed herein. The fuel rod may optionally contain other elementssuch as spacers not containing fissile material, or containing a reducedconcentration of fissile material as compared with the fuel pellets.

With reference to FIG. 2, a nuclear reactor core 10 is diagrammaticallyshown, and comprises an array of fuel rods 2. While the illustrativecore 10 includes only a 10×10 array of 100 fuel rods, a reactor forgenerating electrical power may employ thousands or tens of thousands offuel rods, typically arranged in structural groups called fuelassemblies. For example, one contemplated small modular reactor (SMR)design may include up to 69 fuel assemblies each comprising a 17×17bundle of fuel rods. The fuel rods of each fuel assembly are typicallyheld together by spacer grids welded with guide tubes and upper andlower nozzles or end plates to form the structural skeleton, and a corebasket core former, or other structural support contains the fuelassemblies (structural support components not shown).

With reference to FIG. 3, a nuclear reactor is diagrammatically shown,and includes the nuclear reactor core 10 disposed in a pressure vessel12 and immersed in primary coolant water. In the illustrative reactor ofFIG. 3 a central riser structure 14 defines primary coolant flowcirculation paths 17 which may be driven by natural circulation (thatis, convection currents due to heating by the reactor core 10) or byinternal or external reactor coolant pumps (not shown). The circulationpaths 17 entail primary coolant heated by the reactor core 10 flowingupward through the interior plenum of the central riser 14 (the “hot”leg) and back downward to return to the core 10 via a downcomer annulusdefined between the pressure vessel 12 and central riser 14 (the “cold”leg). The nuclear reactor optionally includes various other componentsnot shown in FIG. 3, such as control rods and associated control roddrive mechanisms (CRDMs), optional reactor coolant pumps, an internal orexternal pressurizer, coolant makeup and letdown sub-systems, emergencycore cooling systems (ECCS), an external containment structure, and soforth.

With reference to FIGS. 4 and 5, an illustrative resistance weldingprocess is shown for welding the lower end plug 8 to the cladding tube4. The lower end plug 8 is generally cylindrical and includes a mainbody comprising a tapered (e.g. conical) end 20, a cylindrical main body22, a cylindrical bonding portion 24, and a cladding seat 26 which inthe embodiment of FIGS. 4 and 5 is a chamfered end of the cylindricalbonding portion 24. In the end plug 8 of FIGS. 4 and 5, the cylindricalmain body 22, cylindrical bonding portion 24, and cladding seat 26comprise three successively smaller cylindrical portions of respectivelysmaller diameters D₁>D₂>D₃ extending between the tapered end 20 at oneend of the end plug 8 and the cladding seat 26 at the opposite end ofthe end plug 8. The cladding seat 26 has a reduced diameter as comparedwith the diameter D₂ of the cylindrical bonding portion 24, e.g.comprising a chamfered or frustoconical end of the end plug 8. Thereduced-diameter cladding seat 26 is sized to be received fully (asshown in FIG. 4) or partway into an open end of the cladding tube 4 thatis to be plugged by the end plug 8. The chamfered or frustoconical endof the cladding seat 26 provides alignment and lead-in for the claddingtube 4 during the resistance welding. The cylindrical bonding portion 24is sized to form an interference fit within the cladding tube 4. To thisend, the cylindrical bonding portion 24 has the same diameter orslightly larger diameter than the inside diameter D_(C,ID) of the end ofthe cladding tube 4. In some embodiments, the diameter D₂ of thecylindrical bonding portion 24 has a diameter that is 0.002 to 0.004inches larger than the inside diameter D_(C,ID) of the end of thecladding tube 4.

To perform the resistance welding, an electrode is attached to each ofthe end plug 8 and the cladding tube 4. In illustrative FIGS. 4 and 5, acladding electrode 30 is a clamshell-type electrode that clamps onto theend of the cladding tube 4. In an alternative configuration, thecladding electrode 30 can be sized to engage the cladding tube end 4with an interference fit between the cladding electrode 30 and thecladding tube 4 securing the cladding tube 4 in the electrode 30. Theinterference fit can be on the order of 0.0002 to 0.0020 inches, forexample. An end plug electrode 34 can include a divot or other matingrecess that is configured to receive the tapered end 20 of the end plug8.

With the cladding tube 4 and end plug 8 attached to their respectiveelectrodes 30, 34, the open end of the cladding tube 4 is clampedagainst the end plug 8. The clamping is diagrammatically indicated inFIG. 4 by arrows F. With the clamping F applied, an electrical currentis applied between the electrodes 30, 34 and flows between the claddingtube 4 and the end plug 8 while the cladding tube 4 and end plug 8 arecompressively loaded or clamped together. Electrical resistance isexpected to be highest at the interface between the cladding tube 4 andthe end plug 8. This resistance causes resistive heating of the matingsurfaces. The combination of the clamping F and the applied electriccurrent is effective to force the open end of the cladding tube 4 overthe cladding seat 26 and to slide over the cylindrical bond portion 24of the end plug 8 and to generate a resistance weld 25 (see FIG. 5)between a cylindrical bonding surface of the cylindrical bond portion 24of the end plug 8 and the inside surface of the open end of the claddingtube 4. To achieve this effect, the clamping force F is approximately200-800 pounds in some embodiments, for example. The combination of thecompressive force and heating of the components results in the insidediameter of the cladding tube 4 sliding over the outside diameter D₂ ofthe cylindrical bond portion 24 of the end plug 8. To facilitate this,the inside diameter of the cladding tube 4 is urged radially outwardlyover the cylindrical bond surface (24). The heating due to theelectrical resistance at the plug/cladding interface causes the metal tosoften and, under the compressive force, the cylindrical bond portion 24of the end plug 8 slides into the cladding tube 20. The slidingcontinues until the cladding tube 4 reaches the radially outwardlyextending shoulder 38 of the end plug 8 between the smaller diameterbonding surface 24 and a larger diameter cylindrical surface of the mainbody 22 of the end plug 8. The shoulder 38 is thus a slide stop 38defined between the main body 22 of the end plug 8 and the cylindricalbond portion 24 of the end plug. The slide of the open end of thecladding tube 4 over the cylindrical bond portion 24 of the end plug 8is stopped by the slide stop. Alternatively, the slide of the tube overthe end plug may be controlled by limiting the travel of the plug or bythe process itself.

The diameter D₁ of the main body 22 of the end plug 8 is preferably onlyslightly larger than the diameter D₂ of the cylindrical bond portion 24.In some embodiments the diameter D₁ of the main body 22 of the end plug8 is equal to an outer diameter D_(C,OD) of the cladding 4, so that inthe welded configuration (FIG. 5) the outer surface of the fuel rod hasthe same outer diameter across the transition from the cladding 4 to thewelded end plug 8.

As the cladding tube 4 slides over the cylindrical bond portion 24 ofthe end plug 8, some material, from the end plug 8 and/or from thecladding tube 4, builds up at the leading edge of the interface betweenthe two components. This buildup 44 is shown diagrammatically in FIG. 5.In some embodiments (see FIGS. 8 and 9), a groove in the end plug 8 (forexample, at the location identified generally by reference numeral 44)is provided to accept the buildup, and/or a chamfer can be added to theradially outwardly extending seating shoulder to urge upset materialoutwardly. In general, the welding process is self-cleaning asimpurities are displaced with the upset material (i.e., buildup 44) asthe cladding slides over the cylindrical bond portion 24 of the end plug8. Moreover, the bond length of the weld 25 is large, e.g. correspondingto the axial length L_(w) of the cylindrical bond portion 24 asindicated in FIG. 5.

With reference to FIGS. 6-9, other illustrative end plug configurationsare illustrated, which include a cladding seat and cylindrical bondportion and are generally suitable for welding to cladding tubes in asimilar manner as the end plug 8 shown and described in FIGS. 4-5. Inaddition, the various features of each end plug are generallyinterchangeable such that features shown in one embodiment can beincluded in other embodiments as desired. The end plugs of FIGS. 6-9each include the main body comprising the tapered (e.g. conical) end 20and cylindrical main body 22. Upper end plugs (not show) may include anyupper end plug upper end portion in combination with any (one or more)of the main body, bonding, seat, recess, slide stop, or stub featuresdisclosed herein with respect to a lower end plug feature. The claddingseat of each of the embodiments of FIG. 6-9 is modified versus the endplug 8 of FIGS. 4-5 to include a stub 48. The stub 48 is an optionalfeature that, if included, provides a connection point for connecting aspacer element (not shown) for spacing the fuel pellets 6 (see FIG. 1)from the end plug.

With reference to FIG. 6, an end plug 50 is illustrated that is similarto end plug 8, and includes the conical end 20, cylindrical main body22, and cylindrical bond portion 24 with reduced diameter compared withthe main body 22, and the slide stop 38 defined as an abrupt stepbetween the main body 22 and the cylindrical bond portion 24. However,the cladding seat 26 of the embodiment of FIGS. 4 and 5, which has anabrupt reduction from diameter D₂ to diameter D₃, is replaced in the endplug 50 of FIG. 6 by a modified cladding seat 56 comprising afrustoconical or chamfered structure having gradually reducing diameterwith increasing distance from the cylindrical bond portion 24. Thechamfered or frustoconical shape of the cladding seat 56 facilitatesurging the cladding radially outward as it is forced over the claddingseat 56 and slides onto the cylindrical bond portion 24 during theresistance welding. At the point where the cladding seat 56 joins thecylindrical bond portion 24, the radius of the cladding seat 56 shouldbe equal to or slightly smaller than the diameter of the cylindricalbond portion 24 but should also be larger than the inner diameterD_(C,ID) of the cladding 4 so as to capture and seat the end of thecladding 4.

With reference to FIG. 7, an illustrative end plug 60 is illustratedthat is similar to end plug 50 of FIG. 6, and includes the conical end20, cylindrical main body 22, cylindrical bond portion 24 with reduceddiameter compared with the main body 22, and chamfered or frustoconicalcladding seat 56. However, the end plug 60 of FIG. 7 differs from theend plug 50 of FIG. 6 in that the slide stop 38, which is an abrupt stepbetween the main body 22 and the cylindrical bond portion 24 in theembodiments of FIGS. 4-5 and 6, is a gradual-step slide stop 68 in theembodiment of FIG. 7. The illustrative gradual-step slide stop 68 is a45° chamfer; however, other angles are contemplated. The gradual slidestop 68 advantageously provides space for the buildup. In contrast, theabrupt-edge slide stop 38 of FIGS. 4-5 and 6 can act as a trap for thebuildup.

With reference to FIG. 8, an illustrative end plug 70 is illustratedthat is similar to end plug 50 of FIG. 6, and includes the conical end20, cylindrical main body 22, cylindrical bond portion 24 with reduceddiameter compared with the main body 22 defining the abrupt-step slidestop 38, and chamfered or frustoconical cladding seat 56. However, theend plug 70 of FIG. 8 differs from the end plug 50 of FIG. 6 in that anannular recess or groove 79 is provided at the base of the abrupt-stepslide stop 38 to receive buildup generated during the resistance weldingprocess. The annular recess or groove 79 is located proximate to wherethe leading edge of the cladding tube 4 meets the slide stop 38 toaccept the buildup. The recess or groove 79 reduces the outward “pileup”of the buildup on the exterior of the fuel rod. Any remaining excessfiller can be removed mechanically.

With reference to FIG. 9, an illustrative end plug 80 is a composite ofthe end plugs 70, 80 of FIGS. 7 and 8, and includes the conical end 20,cylindrical main body 22, cylindrical bond portion 24 with reduceddiameter compared with the main body 22 defining the gradual-step slidestop 68 (as in the end plug 60 of FIG. 7), and chamfered orfrustoconical cladding seat 56, and further includes the recess orgroove 79 (as in the end plug 70 of FIG. 8).

The abrupt-step slide stop 38 or gradual-step slide stop 68advantageously provides a definite termination for the slide of thecladding 4 over the cylindrical bond portion 24. However, it iscontemplated to omit the slide stop entirely. In such embodiments, theslide of the cladding 4 over the cylindrical bond portion 24 isterminated due to force from an interference fit between the cladding 4and the cylindrical bond portion 24, possibly in combination withincipient weld interfacing forming between the cladding 4 and thecylindrical bond portion 24 as the sliding increase the total contactsurface area resulting in reduced resistive heating. The resistancewelding is a dynamic process that is difficult to model. Accordingly,determination of process parameters such as the inside diameter D_(C,ID)of the cladding 4, the outer diameter D₂ of the cylindrical bondingportion 24, the clamping force F, and the electrical current suitable toproduce the desired slide stopping force is contemplated to bedetermined by trial-and-error using an experimental resistance weldingapparatus.

The resistance welding process disclosed herein has numerous advantages.For example, the welding process is self-cleaning as impurities aredisplaced with the buildup material as the cladding slides over thecylindrical bonding portion 24 of the end plug. In addition, the bondlength L_(w) can be significantly longer than the cladding tubethickness (e.g., twice as long or more), providing a strong weld. Theresulting bond line is parallel to the surface of the cladding 4, whichcan facilitate non-destructive examination of the weld bond. Theresulting weld does not protrude above the cladding surface, and in someembodiments no post-weld machining is performed to smooth the surface.(Alternatively, if the gradual-step slide stop 68 and/or groove 79 isinsufficient to fully contain the buildup, some post-weld machining toremove the excess buildup is contemplated). The weld quality can bemonitored and controlled on the basis of welding process parameters suchas electrical current, voltage, displacement, weld time and weldclamping force. Moreover, the abrupt-step cladding seat 26 or thegradual-step (e.g., chamfered or frustoconical) cladding seat 56provides secure and well-centered seating of the cladding 4 against theend of the end plug, which substantially enhances robustness,reliability, and yield. The precise centering of the cladding seatingalso facilitates the use of the illustrative spacer connection stub 48,as the centering of the cladding ensures that the cladding does not comeinto contact with or interfere with the positioning of the spacer on thestub 48. The stub 48 is suitably sized and shaped to not contact theopen end of the cladding tube 4 during the plugging process (i.e., asthe end plug is resistance welded to the cladding tube 4).

The preferred embodiments have been illustrated and described.Obviously, modifications and alterations will occur to others uponreading and understanding the preceding detailed description. It isintended that the invention be construed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

We claim:
 1. An apparatus comprising: a nuclear fuel rod including: anend plug having a cylindrical bond portion, a tapered tip at one end ofthe end plug, and a seat portion of reduced diameter compared with thecylindrical bond portion at the opposite end of the end plug; a claddingtube having an end plugged by the cylindrical bond portion of the endplug with the seat portion disposed inside the plugged end of thecladding tube and the tapered tip extending outside of the plugged endof the cladding tube; and a weld bonding the cylindrical bond portion ofthe end plug and the inside surface of the plugged end of the claddingtube.
 2. The apparatus of claim 1 wherein the weld is a resistance weld.3. The apparatus of claim 1 wherein the seat portion comprises acylindrical seat portion of reduced diameter compared with thecylindrical bond portion.
 4. The apparatus of claim 1 wherein the seatportion comprises a chamfered or frustoconical end of the cylindricalbond portion of the end plug.
 5. The apparatus of claim 1 wherein theend plug further has a cylindrical main body disposed between thecylindrical bond portion and the tapered tip, the cylindrical main bodyextending outside of the plugged end of the cladding tube andterminating in the tapered tip.
 6. The apparatus of claim 5 wherein thecylindrical main body has a larger diameter than the cylindrical bondportion and an abrupt or gradual annular step is defined between thecylindrical main body of the end plug and the cylindrical bond portionof the end plug.
 7. The apparatus of claim 6 wherein the end plugfurther has an annular groove in the cylindrical bond portion at theabrupt or gradual annular step defined between the cylindrical main bodyand the cylindrical bond portion.
 8. The apparatus of claim 7 whereinthe annular groove contains buildup material generated during formationof the weld.
 9. The apparatus of claim 1 wherein the end plug furtherincludes a stub protruding from the seat portion, the stub notcontacting the cladding tube.
 10. The apparatus of claim 1 wherein thenuclear fuel rod further includes a stack of fuel pellets comprisingfissile material disposed in the cladding tube.
 11. The apparatus ofclaim 10 further comprising: a pressure vessel containing a nuclearreactor core comprising an array of said nuclear fuel rods immersed inprimary coolant water.
 12. An apparatus comprising: an end plug forplugging an end of a nuclear fuel rod cladding tube, the end plugincluding: a tapered tip at one end of the end plug; a cylindrical bondportion sized to form an interference fit inside the end of the nuclearfuel rod cladding tube; and a seat portion of reduced diameter comparedwith the cylindrical bond portion at the opposite end of the end plugfrom the tapered tip.
 13. The apparatus of claim 12 wherein the end plugfurther includes a cylindrical main body of diameter D₁ disposed betweenthe tapered tip and the cylindrical bond portion, and the cylindricalbond portion has diameter D₂ which is less than diameter D₁ of thecylindrical main body.
 14. The apparatus of claim 13 wherein the seatportion comprises a cylindrical seat portion having diameter D₃ which isless than diameter D₂ of the cylindrical bond portion.
 15. The apparatusof claim 13 wherein the seat portion comprises a chamfered orfrustoconical end of the cylindrical seat portion.